1. Feature Of The Week

    Industrial monuments and other artefacts of vanishing industries have to be preserved as cultural heritage. For example, in Germany next year the public subsidy for the mining industry will end and with this the active mining will end as well. The Deutsches Bergbau-Museum in Bochum as one of the largest of its kind and as a research institution of the Leibnitz association is committed to preserve the cultural heritage of the mining industry. Thus it is facing the huge task to conserve a lot of mining industry objects in their actual authentic appearance, ranging from small tools like hammers to huge outdoor facilities like e.g. a blast furnace. Common non-transparent protective coatings are not applicable, because the historical character of these cultural heritages must be maintained.

    Therefore alternative transparent coatings have to be evaluated with respect to their functionality and durability. For the in-situ monitoring of real historical artefacts the tools need to be non-destructive and easy to use.

    The current standard method to evaluate anti-corrosion coatings for metallic substrates is Electrochemical Impedance Spectroscopy. Here a probe consisting of an electrolyte with two electrodes is contacted with the sample. An alternating voltage with a swept frequency is applied and the current is measured. Information about the protection efficiency of the coating and about the underlying corrosion process can be derived indirectly by fitting the curves with equivalent electrical circuit models of the sample and the total setup. But this procedure is rather complex, bears the risk of ambiguity and of course does not provide a direct visual impression. Furthermore quantitative values e.g. for the actual layer thickness are not provided and the measurement procedure itself is very time consuming.

    A method to directly measure the thickness of a non-magnetic coating on a magnetic substrate is the magnetic induction technique. A magnetic probe is contacted with the sample surface and an alternating magnetic field is applied. With a second coil the retarded magnetic flux is measured. The induced voltage correlates with distance of the probe to the metallic substrates that is with the thickness of the coating layer. The coating thicknesses can be measured with an accuracy of about 1 µm. The lateral resolution depends on the probe size and is about 1 mm2. But this technique fails on pre-corroded samples which is a drawback for our work, since we are dealing exactly with that kind of samples.

    In this work we present optical coherence tomography (OCT) as a new diagnostic tool for the evaluation of transparent anti-corrosion coatings for the conservation of industrial heritage.

    Since OCT provides three dimensional cross-sectional images the actual coating layer thickness can be determined. Moreover, by introducing a scattering layer and measuring its thickness a metric for the differentiation between metal and corrosion as underlying substrate can be derived.

    First the accuracy of our coating thickness measurements was validated by comparing the OCT results with magnetic inductive measurements on six samples with bare metal and varying coating layer thicknesses. Additionally a customized sectioned sample was fabricated. With this sample, we demonstrated that OCT is capable of monitoring the coating layer thickness even for corroded areas, and that regions with metal or corrosion can be separated. Moreover, a standard sample with randomly distributed corrosion was investigated to show, that OCT is also capable to monitor real world samples with no defined boundaries.

    With this work, we have illustrated the benefit of employing optical coherence tomography for the non-destructive investigation of transparent protection coatings. To our knowledge, no other non-destructive modality has provided comparable information. Moreover, by using OCT, the differentiation between metal and corroded material becomes possible with µm resolution in the lateral dimension.

    Acknowledgments: This work was financially supported by the Stiftung Rheinisch-Westfälischer Technischer Überwachungsverein (RWTÜV, Essen, Germany) (Project Numbers: S189/10024/2015, S189/10025/2015). We acknowledge support by the DFG Open Access Publication Funds of the Ruhr-Universität Bochum (Bochum, Germany).

    For more information see recent Article. Courtesy Martin Hofmann from Ruhr-University Bochum. To share this article click Here.

  2. Recent Articles From Around The Web

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